1. Field of the Invention
This invention relates to stators for permanent magnet commutator type D.C. electric servomotors and, more particularly, to a new and improved optimization of the stator interpole area to improve high torque performance capability of such D.C. electric servomotors.
2. Description of the Prior Art
Most permanent magnet D.C. electric servomotors include a stator fabricated from a tube manufactured in one of a variety of methods to which are added the permanent magnets. Such tubing in such servomotors is usually cylindrical with an essentially uniform wall thickness. Permanent magnets in groups of multiples of two are equally spaced symmetrically with respect to the pole centerline and are held in fixed position by mechanical means, such as by clamps or with an adhesive. The thickness of the magnets, radial of the pole centerline, is determined by the magnet material necessary to maintain sufficient magnetic saturation in the servomotor magnetic circuit. The passive magnetic tube which, with the permanent magnets, makes up the stator, may be continuous or, as is more common, may be a laminate of a plurality of coaxial discs which, in the lamination, form the tube.
Advances in technology have increased the need for higher motor torques. Attempts, heretofore, to improve high torque motor performance have met with limited success. Improvements have made stronger magnets available and made possible the use of thinner magnets. The result has been a reduction in interpole spacing between the yoke and armature. The result is increased interpole leakage which induces voltage in armature coils being commutated which results in excessive sparking at the brushes and increases the risk of flashover at the commutator. This leakage is the result of armature reaction. In addition, in one region of the stator the armature field opposes the stator flux while, in another region, the armature field aids the stator flux. The net result, because of steel saturation, is a reduction in overall stator flux.
The above described problem is particularly pronounced when rare earth (RECO.sub.5) magnets with an energy product of 18 MGO are used in the stator in combination with a conventional wound iron core armature. These rare earth magnets are highly resistant to flux reversals. In order for a permanent magnet commutator type D.C. electric servomotor utilizing these rare earth magnets to accelerate and decelerate quickly without sacrificing performance, an expensive cup type armature is necessary in lieu of the conventional wound iron core armature. A typical example of a permanent magnet motor having a cup type armature is described in U.S. Pat. No. 3,102,964 by J. L. Bennett, et al. These cup type armatures are relatively difficult to manufacture and add considerably to the cost of the motor.
Accordingly, there is a need for a permanent magnet commutator type D.C. electric servomotor having a low cost conventional wound iron core armature which can accelerate and decelerate quickly without sacrificing performance.